Tape drives, such as, for example, digital data storage (DDS) tape drives are commonly used to back up and record information in the form of digital data from a computer system. A DDS tape drive, for example, is a helical scanning tape drive that writes and reads information in the form of digital data to and from a magnetic tape. DDS tape drives provide a low cost storage mechanism that is light weight, compact and typically very reliable. DDS tape drives have continued to evolve over time, such that each new generation of DDS drives and DDS formatted tapes provides additional storage capacity over the earlier generations.
One advantage of DDS tape drives over linear tape drives is the use of a helical scanning method that allows very high recording densities. Basically, a helical scanning method, such as, for example, in a DDS tape drive, uses a rapidly rotating scanner that has two read heads and two write heads (i.e., for a total of four transducers). The rotating scanner is tilted at an angle in relation to the horizontal movement of the tape, which is being transported at a given speed, and the tape is wrapped about at least a portion of the scanner. Thus, the horizontal tape movement against the tilted and rotating scanner causes diagonally positioned tracks to be written to the tape (and subsequently read from the tape). The speed and tension of the tape are typically kept constant by a tape drive servoing system that includes controlling circuitry and mechanical mechanisms, such as, for example, a capstan and series of rollers and guides.
The format of the recorded data tracks (e.g., containing raw, compressed, timing, control and/or error correction data) on a DDS formatted tape, for example, is mandated by a specific DDS standard.
During a read operation of a DDS formatted tape, it is essential to match the tracks as laid down during the previous write operation with the read heads located on the scanner so as to read the data within each track. This is typically done by calibrating the speed at which the tape is transported to properly align the read heads of the scanner to the previously recorded tracks.
Earlier DDS standards, such as, for example, the DDS1 and DDS2 formats, use an automatic track finding (ATF) technique to calibrate the scanner to the tracks. In an ATF formatted tape there are a plurality of sub code areas recorded within each track that can be detected and used to determine if a read head is properly aligned over the track. Thus, for example, in a DDS2 formatted tape, the resulting signals from the read head scanning different sub code areas are used to determine if the tracking is proper (i.e., that the read head is centered over the track). Thus, based on the sub codes recorded in the tracks of the DDS2 tape, the speed of the tape within the DDS tape drive is adjusted such that the read head is properly aligned with the recorded tracks. As such, several sub codes are typically required in the earlier DDS formatted tapes. For example, a DDS2 formatted tape includes eight sub codes at the top of each track and eight sub codes at the bottom of each track.
One drawback to an ATF formatted tape, such as DDS2 formatted tape, is the number and size of the sub codes and the amount of space on the recording tape that is required to record these sub codes.
Moreover, because there is a continuing effort to increase the amount of data storage capacity in the DDS tape drive family, the next generation standard format, namely a DDS3 format, does not include ATF. As a result, timing and tracking has to be accomplished through different techniques.
The DDS3 format does not specify how the timing and tracking of a read operation is to be accomplished within the tape drive. Instead this decision has been left to the DDS tape drive design community. Thus, there is a need for efficient methods and apparatus for calibrating and controlling the timing and tracking in a DDS tape drive capable of supporting a DDS formatted tape that does not include ATF information.